TheSamba.com :: View topic - D-Jet fuel trim adjustment

Tue Feb 23, 2021 1:50 pm

We all know the computer of a DJET system runs based on various inputs, mostly Ohm readings. And spits out a fuel output pulse to the injectors.

One of those inputs would be the CHT sensor on the cylinder head.

I'm not exactly sure HOW much the CHT controls of the overall fuel trim, but it does control a small portion of it (same with the air intake temp sensor)

We know the MPS does the "bulk" of the fuel delivery work.

I found this product on Ebay would be interesting to install on a Type 3 cylinder head temp sensor.

It's made for the Djet system of a Porsche 914 which is nearly identical to the Type 3 system.

https://www.ebay.com/itm/Porsche-914-D-Jetronic-ai...1438.l2649

Per the ebay description:

For sale is a fuel-air mixture trim kit which allows you to richen the mixture on the Porsche 914 D-Jetronic fuel injection system from the cockpit of the car. While designed for the Porsche 914, it will work on any car that runs the D-Jet system.

It is important to note that this kit will not fix underlying problems with your fuel injection system. Your D-Jet must be well-sorted already to benefit from this add-on, with all components working properly and zero vacuum leaks.

D-Jet is an excellent 50 year old fuel injection system, but it’s not perfect. This kit is designed to compensate for several shortcomings by adding variable ballast on the cylinder head temperature circuit to richen the mixture.

This kit is designed to address the following:

1. 1. When starting the engine cold, the idle will not come up when the auxiliary air regulator is open because the cold mixture is too lean.

2. 2. The car “hunts” at idle – the engine revs then nearly stalls with the mixture control knob on the ECU full clockwise. The mixture is too lean and there is no remaining adjustment on the ECU.

3. 3. Idle bog down from additional load (headlights, blower etc.).

4. 4. “Heat soak” when you drive somewhere and park. After sitting for a few minutes, the car may become difficult to start, particularly in hot weather. It was long thought that the D-jet was overly rich in this condition, but it has since been proven that this is actually a very lean condition.

Additionally, for those using wideband O2 sensors and an A/F gauge to dial in the mixture on the D-Jet system, this kit allows you to adjust fuel-air mixture on the fly to all operating conditions if you set the base mixture on the lean side of the range.

I have tested this kit on my 914 for the past two years - it works well.

Wed Feb 24, 2021 12:51 am

Mr.Duncan wrote: We all know the computer of a DJET system runs based on various inputs, mostly Ohm readings. And spits out a fuel output pulse to the injectors.

One of those inputs would be the CHT sensor on the cylinder head.

I'm not exactly sure HOW much the CHT controls of the overall fuel trim, but it does control a small portion of it (same with the air intake temp sensor)

We know the MPS does the "bulk" of the fuel delivery work.

I found this product on Ebay would be interesting to install on a Type 3 cylinder head temp sensor.

It's made for the Djet system of a Porsche 914 which is nearly identical to the Type 3 system.

https://www.ebay.com/itm/Porsche-914-D-Jetronic-ai...1438.l2649

Per the ebay description:

For sale is a fuel-air mixture trim kit which allows you to richen the mixture on the Porsche 914 D-Jetronic fuel injection system from the cockpit of the car. While designed for the Porsche 914, it will work on any car that runs the D-Jet system.

It is important to note that this kit will not fix underlying problems with your fuel injection system. Your D-Jet must be well-sorted already to benefit from this add-on, with all components working properly and zero vacuum leaks.

D-Jet is an excellent 50 year old fuel injection system, but it’s not perfect. This kit is designed to compensate for several shortcomings by adding variable ballast on the cylinder head temperature circuit to richen the mixture.

This kit is designed to address the following:

1. 1. When starting the engine cold, the idle will not come up when the auxiliary air regulator is open because the cold mixture is too lean.

2. 2. The car “hunts” at idle – the engine revs then nearly stalls with the mixture control knob on the ECU full clockwise. The mixture is too lean and there is no remaining adjustment on the ECU.

3. 3. Idle bog down from additional load (headlights, blower etc.).

4. 4. “Heat soak” when you drive somewhere and park. After sitting for a few minutes, the car may become difficult to start, particularly in hot weather. It was long thought that the D-jet was overly rich in this condition, but it has since been proven that this is actually a very lean condition.

Additionally, for those using wideband O2 sensors and an A/F gauge to dial in the mixture on the D-Jet system, this kit allows you to adjust fuel-air mixture on the fly to all operating conditions if you set the base mixture on the lean side of the range.

I have tested this kit on my 914 for the past two years - it works well.

Bluntly....no.

Does it work?...kind of. It is just a variable resistor.

The CHT (TS-2) on D-jet (type 3, type 4 in 411/412 and 914 and others).....this sensor only controls about 10-15% of the total fuel "dosage"...maximum. Typically its about 10%.

There are a whole slew of collective fine tuning issues that add to and create that list of problems.

The "heat soak" issue is one of the main causes of hot starting issues and some fine running and idling issues....at specific ambient temperature ranges.

The CHT problem...is that the ECU's in certain models and years....really have no more to gibe of that total of 10% range...when the CHT goes below about 200 ohms. And...the CHT will typically during the hottest running...drop all the way to about 75-100 ohms.

What this means is that the engine and oil will cool off over about the 30 minutes while you are grocery shopping...but the heads...being a big lump of metal stay hot. The engine is now cool enough to say...need about 1-3% enrichment.....and could probably give you that if the CHT had cooled off at he same rate as the engine and maybe gave you 400-500 ohms. But no...its only cooled off enough to move it about 150 ohms....so its still down around 200 ohms. Below the threshold where the ECU will give you a little extra fuel.

Just put a 100 to 150 ohm resistor in line. Done.

So you ask....what...what heppens now with that extra 1t50 ohms of resistance when teh cars cold? Am I not too rich right now?

Yeah...just properly adjust your MPS. Then fix all of your grounds, properly adjust TVS and take all the slop out of your distributor...and geta better coil and a points module while you are at it...and this thing will mysteriously run like a new car.

I'm not saying its a bad product. Its just a variable resistor that acts on a sliver of 10% of your fuel mixture dosage. I simply installed a variable resistor in mine that cost about $2.

This is one of many that are available and all look similar

https://www.alliedelec.com/product/bourns/3296w-1-...lsrc=aw.ds

Its about 1/8" thick, 1/2" long and 1/2" wide. Its about 50 to 1000 ohms maximum and has infinite adjustment across 25 rotations of the screw.

$99.99?...I think not.

Ray

Wed Feb 24, 2021 6:45 am

I would say the CHT makes a huge impact on fuel enrichment. I say this because a month or two back I was having cold start issues and it got to the point where the car couldn't start because the FI system was flooding the engine to where it couldn't start as soon as you turned the key. I found out the CHT was reading was off of what it should have been and was severly enriching fuel upon start up to where it couldn't start. I replaced the CHT and it was fine

Wed Feb 24, 2021 10:16 am

Ceckert64 wrote: I would say the CHT makes a huge impact on fuel enrichment. I say this because a month or two back I was having cold start issues and it got to the point where the car couldn't start because the FI system was flooding the engine to where it couldn't start as soon as you turned the key. I found out the CHT was reading was off of what it should have been and was severly enriching fuel upon start up to where it couldn't start. I replaced the CHT and it was fine

True, if the CHT sensor fails in a big way, it will have a big effect. But that's not the same as saying a fine trim makes a huge difference.

There is a story of a seller of a car that would not start letting it go for a song, and the Samba buyer pulled it off the trailer a mile away, fixed the wire to the CHT, and drove it the rest of the way home.

I will wire up a fixed 200 ohm resistor as a temp fix on the road should the sensor or wire fail. It will have the connector and ring for a convenient hookup on the side of the road. It may idle rough when cold but it will get me home.

Wed Feb 24, 2021 12:55 pm

Ceckert64 wrote: I would say the CHT makes a huge impact on fuel enrichment. I say this because a month or two back I was having cold start issues and it got to the point where the car couldn't start because the FI system was flooding the engine to where it couldn't start as soon as you turned the key. I found out the CHT was reading was off of what it should have been and was severly enriching fuel upon start up to where it couldn't start. I replaced the CHT and it was fine

Yes and no. Again....it does NOT wholesale control your fuel mixture. The CHT is not CAPABLE of flooding your engine.

Let me explain that.....

the CHT can add or subtract across a range of only about 10-15%. Yes...that is significant....in that if during cold start you are missing 5-7%% of the fuel you need....it may be hard to start.

If it does not have the fuel baseline to start...its not going to start....and with D-jet....ANY repeated cranking when cold...when your engine is producing virtually no vacuum.....WILL result in flooding because the MPS is sensing maximum enrichment. Low or no vacuum acts like the throttle is open....to the MPS

Thats what "FLOODS" your engine....not the CHT.

The belief that the CHT or extra ballast resistance is going to be able to give your engine so much fuel that it results in "wet plugs".....is incorrect.

Likewise the belief that adding more resistance to the CHT ...say over the 2500 to 3000 ohms these are when stone cold.....is going to give you enrichment capability as long as you keep adding resistance...is also 100% incorrect.

The CHT is a correcting factor. Its part of the baseline fuel setting...just like the TS-1....which is also only adding a range of about 15%...or a +/- of about 5-7%.

The MPS is the main load sensing device. The CHT....does not even have useful response time. Its not designed to CONTROL your primary fuel mixture in real time. Its far too slow...like about 100X too slow.

While I do believe its a good thing to be able to ballast and trim the response of the CHT/TS-2....because its a bit defective in design.....i.e.....there is no functional usefulness in allowing the CHT to drop below about 125 to 150 ohms...and that factor is THE primary cause of hot start issues (coupled with other fuel mixture tuning issues)......being able to slightly tune the CHT is one thing....and its simple and cheap.

Its also not something that should be needed to be done on a regular basis from the dash.

On tightly tuned D-jet systems....meaning leaner settings, higher compression and better cams and ignitions.....these are the only ones we should be seeing issues with. Once you have done ALL OF THE OTHER THINGS that should be done on an engine like this......ballasting the CHT is something that is done about twice a year when the overall weather makes large ambient temperature changes.

In stock-ish cars this is usually done simply by plugging in or unplugging the TS-1`.....and this is usually POINTING TO.....that you have issues in the MAIN fuel baseline adjustment at the MPS...and or poor fuel pressure stability.

I have found that most D-jet owners who seem to have the need to constantly dial in changes to the CHT other than a baseline ballast.. to keep it from dropping below 100-125 ohms....an/or feel the constant need to tweak CHT ballast with a product like this......in virtually all cases....the REAL ROOT CAUSE of the variable air fuel issue....is with with MPS adjustment or fuel pressure stability...or vacuum control.

Excessive and constant need to tweak TS-2....is a band aid for proper tuning. Ray

Wed Feb 24, 2021 3:20 pm

KTPhil wrote:

True, if the CHT sensor fails in a big way, it will have a big effect. But that's not the same as saying a fine trim makes a huge difference.

There is a story of a seller of a car that would not start letting it go for a song, and the Samba buyer pulled it off the trailer a mile away, fixed the wire to the CHT, and drove it the rest of the way home.

I will wire up a fixed 200 ohm resistor as a temp fix on the road should the sensor or wire fail. It will have the connector and ring for a convenient hookup on the side of the road. It may idle rough when cold but it will get me home.

Russ bought 2 cars where the main problem turned out to be the main FI relay ground screw. One he bought off the dealer's lot (he fixed it after purchase) after tightening the screw, and another from a Samba member that he tightened the screw and drove it off his trailer at home.

Wed Feb 24, 2021 4:41 pm

Bobnotch wrote: KTPhil wrote:

True, if the CHT sensor fails in a big way, it will have a big effect. But that's not the same as saying a fine trim makes a huge difference.

There is a story of a seller of a car that would not start letting it go for a song, and the Samba buyer pulled it off the trailer a mile away, fixed the wire to the CHT, and drove it the rest of the way home.

I will wire up a fixed 200 ohm resistor as a temp fix on the road should the sensor or wire fail. It will have the connector and ring for a convenient hookup on the side of the road. It may idle rough when cold but it will get me home.

Russ bought 2 cars where the main problem turned out to be the main FI relay ground screw. One he bought off the dealer's lot (he fixed it after purchase) after tightening the screw, and another from a Samba member that he tightened the screw and drove it off his trailer at home.

Aaah, that sounds like the experience I was (mis-) remembering! Thrill of victory and agony of defeat!

It's also a lesson for those that trash talk the FI and want to immediately convert to carbs, while a little knowledge can often fix it cheaply.

Wed Feb 24, 2021 6:46 pm

Bobnotch wrote: KTPhil wrote:

True, if the CHT sensor fails in a big way, it will have a big effect. But that's not the same as saying a fine trim makes a huge difference.

There is a story of a seller of a car that would not start letting it go for a song, and the Samba buyer pulled it off the trailer a mile away, fixed the wire to the CHT, and drove it the rest of the way home.

I will wire up a fixed 200 ohm resistor as a temp fix on the road should the sensor or wire fail. It will have the connector and ring for a convenient hookup on the side of the road. It may idle rough when cold but it will get me home.

Russ bought 2 cars where the main problem turned out to be the main FI relay ground screw. One he bought off the dealer's lot (he fixed it after purchase) after tightening the screw, and another from a Samba member that he tightened the screw and drove it off his trailer at home.

Oh yeah!....totally!

On the 411 and 412 cars....the main EFI power relay (same part number, metal shell relay as the fuel pump relay).....IIRC......had a ground wire on some thst went to a ring terminal at the screw that held the relay to the firewall.

Let that get loose.....and its no start....crappy running etc. I fixed it....for a while.....by going to a larger screw. Then that stripped out and got loose. Finally PERMANENTLY fixed it by buying a stainless steel press in panel insert that pressed in from the backside and has lottle gripper teeth so it wont turn.....and used a 6mm screw. Looks like factory. Ray

Thu Feb 25, 2021 2:33 pm

Mr.Duncan wrote: We all know the computer of a DJET system runs based on various inputs, mostly Ohm readings. And spits out a fuel output pulse to the injectors.

One of those inputs would be the CHT sensor on the cylinder head.

I'm not exactly sure HOW much the CHT controls of the overall fuel trim, but it does control a small portion of it (same with the air intake temp sensor)

We know the MPS does the "bulk" of the fuel delivery work.

I found this product on Ebay would be interesting to install on a Type 3 cylinder head temp sensor.

It's made for the Djet system of a Porsche 914 which is nearly identical to the Type 3 system.

https://www.ebay.com/itm/Porsche-914-D-Jetronic-ai...1438.l2649

Per the ebay description:

For sale is a fuel-air mixture trim kit which allows you to richen the mixture on the Porsche 914 D-Jetronic fuel injection system from the cockpit of the car. While designed for the Porsche 914, it will work on any car that runs the D-Jet system.

It is important to note that this kit will not fix underlying problems with your fuel injection system. Your D-Jet must be well-sorted already to benefit from this add-on, with all components working properly and zero vacuum leaks.

D-Jet is an excellent 50 year old fuel injection system, but it’s not perfect. This kit is designed to compensate for several shortcomings by adding variable ballast on the cylinder head temperature circuit to richen the mixture.

This kit is designed to address the following:

1. 1. When starting the engine cold, the idle will not come up when the auxiliary air regulator is open because the cold mixture is too lean.

2. 2. The car “hunts” at idle – the engine revs then nearly stalls with the mixture control knob on the ECU full clockwise. The mixture is too lean and there is no remaining adjustment on the ECU.

3. 3. Idle bog down from additional load (headlights, blower etc.).

4. 4. “Heat soak” when you drive somewhere and park. After sitting for a few minutes, the car may become difficult to start, particularly in hot weather. It was long thought that the D-jet was overly rich in this condition, but it has since been proven that this is actually a very lean condition.

Additionally, for those using wideband O2 sensors and an A/F gauge to dial in the mixture on the D-Jet system, this kit allows you to adjust fuel-air mixture on the fly to all operating conditions if you set the base mixture on the lean side of the range.

I have tested this kit on my 914 for the past two years - it works well.

I was kind of waiting for Ray's reply to this before adding my 2 cents.
I would think that same item could be replaced with a potentiomitor(sp?)for less, that you manually adjust for a given amount of resistance (with a minimum of 200 ohms added). On my own Squareback, it had a 200 ohm resistor added into the harness for the CHT. Please note this car was from Wichita Falls TX (all it's life), and had air con dealer installed (along with 3 engines before I got it including a VW recycled one).

On my old 71 Notch (with new FI wiring harness), I added a 100 ohm resistor for the CHT, but this was mainly for summer use. That car also had a freshly rebuilt engine in it, and I didn't want it to run lean.

Thu Feb 25, 2021 7:37 pm

On one hand...this i not badmouthing the kit/part in question. Its a variable resistor. A potentiometer. Same thing I have on mine...except mine is a little enclosure made from a high beam relay and piece of circuit board with a 25 turn ceramic/metallic resistor (cermet).

I bought it for about $4 if I remember...maybe $6. I think I had $8 and about 45 minutes in the whole thing. I made one just like for experimentation for the TS-1. After getting all other items under control and adjusted...it turns out I di not need the TS1 version.

So....the Kit in question...a nice dash knob...ok. That has some value. I found that same basic knob for about $1.50

https://www.arrow.com/en/products/com-10002/sparkf...rkEALw_wcB

A nice painted metal bracket ...ok...that has some value. Its probably the part the person who made this spent the most time one. Cant tell if its bought or built....call it $10

The potentiometer....small value. Very cheap. That three pin potentiometer I can get from Digi-Key for $1.22. Probably 0-1000 ohm....or similar. It may be a little more difficult to get one in the right turn of arc range...but call it $5

The fact that its in a kit...wires, nuts , screws and connectors...ok...that ha some value. Maybe $7

All in all....about $25 in materials. Call it convenience fee for you not having to ship everything in if you an find it local....$10. Call it $35 + 65% profit....which is normal and fair. Thats $57.75.

If it went for $65.....it would be a fair price for a complete "kit". The price of $99.95...is the "Porsche" owners tax that this part is aimed at. :wink:

However....my whole REAL point....is that the problem we are speaking of....the ballasting of the CHT to compensate for its design flaw of dropping below about 125 ohms when its hot....is NOT something that you would or SHOULD be needing to adjust enough to warrant putting a dedicated knob on the dash.

If you have to adjust this knob or any potentiometer attached to the TS-2/CHT...more than twice a year....you have other problems you should be fixing.

Now....if it were being advertised as a band aid method to say...lean fuel mixture when you are climbing in altitude......I could see that...but I do not think the TS-2 has enough input range to fully adjust for that. The fact of the matter is that the TS-2/CHT is going to be at its leanest anyway when you are warmed up....so its no help for that.

What wuld help for leaning out at altitude is a fifth injector going to a bleed line into the return line that you can ramp up...to bleed off fuel pressure to reduce richness across the board.

Or...maybe we can find one of the L-jet barometric compensators used on the 1.8L 914...and see if it can be tweaked to use on D-jet.... :wink:

Ray

Thu Feb 25, 2021 8:05 pm

For the record I do believe the kit is over priced. Aimed at rich Porsche guys.

However, I did hang out once with the 914' crowd. They were super nice and friendly just like VW folks..

Thu Feb 25, 2021 11:18 pm

Mr.Duncan wrote: For the record I do believe the kit is over priced. Aimed at rich Porsche guys.

However, I did hang out once with the 914' crowd. They were super nice and friendly just like VW folks..

Totally agree! The vast majority of 914 guys are an EXCELLENT crowd. They are firmly between VW and actual Porsche 911 and higher. I think they get taken advantage of by people selling products INTO their world.

Not saying its a bad product...just a tad overpriced. I try to get people to think about why and how often they need something where D-jet is concerned. Ray